The present invention relates to a distance measurement device and more specifically to a passive distance measurement device which determines the distance to a subject using its images.
A distance measurement device conventionally built into cameras is a passive one which determines the distance to a subject using its images. This passive distance measurement device detects the displacement of one of two images of the same subject which are obtained through different optical paths relative to the other and determines the distance to that subject on the principle of triangulation.
The image signals are generally produced by an opto-electric conversion sensor array. The sensor array is not necessarily adapted to receive the images of a main subject alone, but will convert into image signals even images, such as background and the like, other than the main subject to be actually measured.
To minimize the effect of background, various techniques have already been developed. For example, there are known a technique that splits a sensor array (Japanese Unexamined Patent Publication No. 62-148910) and a technique that uses a spot where there is very little change in image signal for focusing. Also, a camera that uses distance measurements at multiple points to decide whether a shooting scene is back light or not is disclosed in Japanese Unexamined Patent Publication No. 62-255925.
However, according to the technique disclosed in Japanese Unexamined Patent Publication No. 62-148910, since a spot of the highest contrast is given priority over others, in back light situations by way of example, distance measurement may be made on the basis of a high-contrast spot on the boundary between the subject and the background. With such a spot where near and distant subjects are mixed, there is the possibility of inaccurate distance measurement.
Moreover, in the technique disclosed in Japanese Unexamined Patent Publication No. 2-135311, since a spot where very little change occurs in image signal is given priority, a sufficient contrast cannot be obtained. Thus, a comparison between two images, i.e., a determination of the distance to the main subject, becomes often impossible.
Furthermore, the technique disclosed in Japanese Unexamined Patent Publication No. 62-255925 contemplates only deciding back light situations, but not taking measures against such a situation where near and distant subjects are mixed as described above at all.